This invention relates to an iron alloy article formed by sintering an iron powder compact comprising carbon and boron additions. More particularly, this invention relates to a product iron article having a microstructure comprising dispersed large borocementite particles that are sized and distributed to improve wear resistance.
It is known to produce an iron article by compacting iron powder and sintering to diffusion bond the iron into a cohesive structure. The compact contains additives to enhance properties in the product. Carbon powder is added to diffuse into the iron to produce a pearlite or martensite microstructure. Copper may be added for precipitation hardening and enhanced dimensional control. U.S. patent application Ser. No. 745,353, filed June 14, 1985, describes an addition of nickel boride to improve toughness by forming retained austenite about pores that inhibits crack formation.
It is desired in powder metallurgical manufacturing to form a compact that is substantially the size and shape of the desired article so that minimal, if any, machining is required to finish the product. Copper and nickel boride form liquid at preferred iron sintering temperatures. This liquid may enhance diffusion of the agent into the iron. However, if present for a prolonged period, the liquid tends to distort the iron skeleton, so that the sintered article does not conform to the compact.
Also, it is desired to improve wear resistance of the product article. In general, wear resistance of cast iron, in particular cast iron containing chromium, is superior to that of powdered iron articles. This is attributed to the presence of large hard phases in cast iron.
It is an object of this invention to provide an improved iron alloy article formed by compacting and sintering a powder mixture composed predominantly of low-carbon iron powder, which article has a predominantly pearlite or martensite microstructure having dispersed therein hard borocementite particles in an amount and size effective to substantially improve wear resistance.
More particularly, it is an object of this invention to provide a powder metallurgical method for forming a wear-resistant iron alloy article by sintering an iron powder compact comprising a carbon additive and a boron additive, which sintering is effective to cause the carbon and the boron to diffuse into the iron structure away from pores and to concentrate at interior regions to form hard borocementite particles.
In a preferred aspect of this invention, it is an object to provide a powder metallurgical method for forming an iron alloy article having improved wear resistance by compacting and sintering an iron powder mixture comprising carbon powder, a metal boride powder and copper powder, which sintering involves formation of a liquid phase for a time sufficient to promote diffusion of the additives into the iron, but not so long as to produce significant skeletal distortion. The boride powder includes a metal that promotes diffusion of carbon and boron into interior regions of the structure. Similarly, copper promotes carbon and boron diffusion into the interior regions. As a result, carbon and boron concentrate at the interior regions to produce hard borocementite particles that substantially improve wear resistance.